1. Field of the Invention
The invention is based on a fuel injection method for an internal combustion engine in which fuel is injected at at least two different high pressures.
2. Description of the Prior Art
One injection system of this type with which this invention is concerned has been disclosed by International Patent Disclosure WO 98/09068, for instance.
For better comprehension of the ensuing description, several terms will first be defined in more detail: In a pressure-controlled fuel injection system, a valve body (such as a nozzle needle) is opened counter to the action of a closing force by the fuel pressure prevailing in the nozzle chamber of an injector, and thus the injection opening is uncovered for an injection of the fuel. The pressure at which fuel emerges from the nozzle chamber into the cylinder is called the injection pressure. The term stroke-controlled fuel injection system is understood in the context of the invention to mean that the opening and closing of the injection opening of an injector takes place with the aid of a displaceable valve member on the basis of the hydraulic cooperation of the fuel pressures in a nozzle chamber and in a control chamber. An arrangement is furthermore described below as central when it is provided jointly for all the cylinders, and as local if it is intended for only a single cylinder.
In the injection system described in WO 98/09068, both the injection at the higher fuel pressure and the injection at the lower fuel pressure are done under stroke control, and the control chamber and nozzle chamber communicate directly with one another. Since the higher injection pressure also prevails in the control chamber, there are corresponding requirements in terms of sealing function, spring forces and a valve member that must be met there as well. The stroke control makes good replicability of the injection at the lower fuel pressure possible.
From European Patent Disclosure EP 0 711 914 A1, a pressure-controlled fuel injection system is known, in which via a valve control unit, either the lower or the higher fuel pressure is carried into the nozzle chamber of the injector. There, by means of the pressure, a spring-loaded valve body is lifted from its valve seat, so that fuel can emerge from the injection opening. In pressure-controlled injection systems, pressure waves are induced in the injection, which although they are wanted in the main injection at the higher fuel pressure can nevertheless, in the pre-injection at the lower fuel pressure, adversely affect the hydraulic behavior of the injection system in the subsequent main injection.
According to the invention, the advantages of a stroke-controlled and a pressure-controlled injection system are combined. This has decisive advantages:
more-flexible pre-injection and post-injection;
better metering options and good replicability of the pre-injection and post-injection by means of a stroke control and a lower injection pressure;
very small structural size of the injector, since the stroke control, because of the low pressure, can have a 2/2-way valve as its control device;
the use of fast-switching magnet valves which require little current;
only slight influence of component tolerances on the pre-injection and post-injection;
pressure exaggeration in the main injection and a triangular course of injection;
lesser demands in terms of sealing function, spring forces and a valve member because of lesser pressure in the pre-injection and post-injection;
the possibility of choosing the injection principle in the main injection at low injection pressures.
The lower fuel pressure can also be used for the main injection, to realize a bootlike course of injection.
Further advantages and advantageous features of the subject of the invention can be learned from the description, drawings and claims.